The present invention relates to a process for eliminating, reducing and/or suppressing halogenated compounds, in particular chlorinated compounds, contained in a gas or a liquid. In some industrial applications, it is necessary to eliminate halogenated compounds, in particular chlorinated compounds, which contaminate the stream, whether it is gaseous or a liquid.
By way of illustration, halogenated compounds, in particular chlorinated compounds contained in the gas or liquid from catalytic reforming, are eliminated in the petroleum industry.
One aim of catalytic reforming is to obtain hydrocarbons with an increased octane number. Hydrocarbon cyclisation and aromatisation reactions are thus desirable since the octane number of a hydrocarbon is higher when it is branched, cyclic or even aromatic. Catalytic reforming produces hydrogen in parallel.
Normally, these hydrocarbon cyclisation and aromatisation reactions take place in the presence of heterogeneous bimetallic chlorinated catalysts. Such chlorinated catalysts are based on alumina and usually comprise platinum and another metal such as tin, rhenium or iridium. The presence of chlorine in said catalysts is important as when added to alumina, it provides the system with an overall acidity and participates in re-dispersing the platinum over time and thus stabilises the catalytic activity of the catalyst.
However, adding chlorine is not a solution without disadvantages. It has been shown that over time, the chlorine partially elutes, in particular in the form of HCl. Such elution means that the catalyst must constantly be recharged with chlorine. It also leads to the presence of HCl and other chlorinated compounds in the gaseous and liquid effluents from catalytic reforming, which can lead to a problem with corrosion of the facility and to the formation of other products which are undesirable and a nuisance to the operation of the facility.
A catalytic reforming process is conventionally operated at high pressure, usually about 20 bars or even higher.
Regenerative processes or new generation processes have recently been developed and are being further developed in the field. Such processes are characterized by being operated at substantially lower pressures, around 3 to 15 bars, or even lower.
The effluents leaving a traditional reforming process contain mainly hydrogen, light saturated hydrocarbons such as methane, ethane. . . , traces of chlorinated compounds, in particular HCl, and water.
The Applicant has surprisingly and unexpectedly discovered that in regenerative processes, traditional adsorbents used up to now in catalytic reforming at higher pressure, based in a high specific surface area alumina, optionally comprising an alkali or alkaline-earth compound, have substantial disadvantages.
Downstream of regenerative processes, and probably because of the lower pressure operation, in addition to hydrogen, light saturated hydrocarbons, traces of chlorinated compounds in particular HCl, and water, the presence of traces of unsaturated hydrocarbons such as ethylene, propylene, butadiene, . . . , has also been observed.
The traces of unsaturated hydrocarbons present in the streams downstream of regenerative processes are at least partially transformed into organo-chlorinated compounds in the presence of chlorinated compounds when in contact with the alumina. Such organo-chlorinated compounds, after multiple reactions with each other and/or with other unsaturated compounds also present in the stream and at the surface of prior art adsorbants, lead to the production of chlorinated oligomers which are aromatic to a greater or lesser extent and have high molecular weights, known as green oils.
After their formation, green oils can no longer be eliminated by traditional adsorbents and migrate in the direction downstream of the adsorber. They can then cause blockages in the facility. Further, a significant drop in the service life of the adsorbent is noted: in some cases, a fall of 4 to 5 times has been observed.
The aim of the present invention is to provide a process which can use a composition which substantially reduces or even suppresses the formation of halogenated oligomers, and in particular chlorinated oligomers known as green oils.
The present invention also aims to provide a process which uses a composition which can more efficiently eliminate halogenated compounds, in particular chlorinated compounds, contained in a gas or a liquid.
These aims are achieved by the present invention which provides a process for eliminating halogenated compounds and for reducing or suppressing the formation of halogenated oligomers contained in a gas or a liquid.
The expression xe2x80x9cprocess for eliminating, reducing and/or suppressing halogenated compoundsxe2x80x9d as used below designates a xe2x80x9cprocess for eliminating, reducing and/or suppressing both organic and inorganic halogenated compounds and high molecular weight halogenated oligomersxe2x80x9d. This expression also designates the process aimed at preventing the formation of the halogenated oligomers cited above.
In a first variation, the invention concerns a process for eliminating, reducing and/or suppressing halogenated compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition obtained by depositing on alumina at least one compound comprising at least one element selected from alkalis on an alumina, followed after deposition by calcining the alumina at a temperature of at least 600xc2x0 C.
In a second variation, the invention concerns a process for eliminating, reducing and/or suppressing halogenated compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition obtained by depositing at least one compound comprising at least one element selected from alkaline-earths and rare earths on an alumina, followed after deposition by calcining the alumina at a temperature of at least 500xc2x0 C., more particularly at least 600xc2x0 C.
In a third variation, the invention concerns a process for eliminating, reducing and/or suppressing halogenated compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition obtained by depositing at least one compound comprising at least one element selected from alkalis on an alumina, followed after deposition by calcining the alumina at a temperature of at least 600xc2x0 C., said composition having a specific surface area of at most 110 m2/g.
In a fourth variation, the invention concerns a process for eliminating, reducing and/or suppressing halogenated compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition obtained by depositing at least one compound comprising at least one element selected from alkalis on an alumina, followed after deposition by calcining the alumina at a temperature of at least 500xc2x0 C., more particularly at least 600xc2x0 C., said composition having a specific surface area of at most 140 m2/g.
Finally, in a fifth variation, the invention concerns a process for eliminating, reducing and/or suppressing halogenated compounds contained in a gas or a liquid, characterized in that the gas or liquid is brought into contact with a composition obtained by depositing at least one compound comprising at least one element selected from the rare earths on an alumina, followed after deposition by calcining the alumina at a temperature of at least 500xc2x0 C., more particularly at least 600xc2x0 C., said composition having a specific surface area of at most 250 m2/g.
The upper temperature for the calcinations of alumina preferably does not 1200xc2x0 C.
The composition used in the process of the present invention can be in a variety of forms. As an example, it can be in the form of powders, beads, extrudates, compresses, or monoliths.
The first essential constituent of the composition is alumina.
The specific surface area of the starting alumina is generally at least 30 m2/g, preferably at least 100 m2/g, more preferably at least 200 m2/g.
In the present invention, all specific surface areas quoted are surface areas measured using the BET method. The term xe2x80x9csurface area measured using the BET methodxe2x80x9d means the specific surface area measured by nitrogen adsorption in accordance with American Standard ASTM D 3663-78 based on the BRUNAUER-EMETT-TELLER method described in xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d, 60, 309, (1938).
This starting alumina also has a total pore volume (TPV) of at least 15 cm3/g, preferably at least 20 cm3/g, still more preferably at least 25 cm3/g. This total pore volume is measured as follows: the value of the grain density and of the absolute density are determined: the grain density (Dg) and absolute density (Da) are measured using the picnometry method with mercury and helium respectively, and the TPV is given by the formula:
[1/Dg]xe2x88x92[1/Da]
Processes for preparing aluminas with the total pore volume and specific surface area features required for carrying out the process of the invention are known to the skilled person.
Regarding alumina, the alumina powder used as the starting material to prepare the composition of the invention can be obtained by conventional processes such as a gel or precipitation method, and a process for rapid dehydration of an alumina hydroxide such as a Bayer hydrate (hydrargillite). This latter alumina is that which is preferred.
Regarding alumina beads, they can be produced by an oil drop process. This type of beads can, for example, be prepared as disclosed in European patents EP-A-0 015 801 or EP-A-0 097 539. The porosity can be controlled using the process descibed in EP-A-0 097 539 by oil drop coagulation of an aqueous suspension or dispersion of alumina or a solution of a basic aluminium salt in the form of an emulsion constituted by an organic phase, an aqueous phase and a surfactant or an emulsifying agent. In particular, said organic phase can be a hydrocarbon, and the surfactant or emulsifying agent is, for example Galoryl EM 10(copyright).
Alumina in bead form can also be obtained by agglomeration of an alumina powder. Agglomeration into beads is carried out directly on the alumina powder using rotary techniques. The term xe2x80x9crotary techniquexe2x80x9d means any apparatus in which agglomeration is carried out by bringing the product to be granulated into rotary contact with itself Examples of apparatus of this type which can be cited are the rotary granulator and the rotary drum. This type of process can produce beads with controlled dimensions and pore distributions, these dimensions and distributions generally being created during the agglomeration step. The pore volume of pores with a given diameter can also be controlled during the agglomeration step by making a suitable adjustment to the rate of introduction of the alumina powder and any water, to the rotation speed of the apparatus or during introduction of a seed.
Alumina extrudates can be obtained by mixing then extruding an alumina-based material, said material possibly being the result of rapid dehydration of hydrargillite or from precipitation of an alumina gel. The porosity of the extrudates can be controlled by the operating conditions used to mix the alumina before extruding. The alumina can also be mixed with pore-forming agents during mixing. As an example, the extrudates can be prepared using the preparation process described in U.S. Pat. No. 3,856,708.
Crushed alumina can be the result of crushing any type of alumina-based material such as beads obtained by any type of process (drop coagulation, rotary granulator or rotary drum) or extrudates. The porosity of such crushed material is controlled by selecting the alumina-based material which is crushed to produce it.
Regardless of the form of the alumina, the porosity can be created by different means such as the choice of the grain size of the alumina powder or the mixture of several alumina powders with different grain sizes. A further method consists of mixing a compound known as a pore-forming agent with the alumina powder before or during the agglomeration or extrusion steps, which agent disappears completely on heating and thus creates pores in the alumina.
Pore-forming agents which can be cited by way of example are wood flour, wood charcoal, sulphur, tars, plastics materials or emulsions of plastics materials such as polyvinyl chloride, polyvinyl alcohols, naphthalene or the like. The quantity of pore-forming agents added is not critical and is determined by the desired pore volume.
Following forming, the alumina obtained can undergo different operations intended to improve its strength, such as maturing by keeping it in an atmosphere with a controlled humidity, followed by calcining then impregnating the alumina with a solution of one or more acids and hydrothermal treatment in a confined atmosphere.
Finally, after these different treatments, the alumina is dried and calcined.
The second essential constituent of the composition is the alkali, alkaline-earth and rare earth element, generally known as the doping element.
The composition of the invention can comprise one element or a combination of alkali and/or alkaline-earth and/or rare earth element(s). Thus in the remainder of the text, any description of a doping element will be applicable to a combination of alkali and/or alkaline-earth and/or rare earth element(s), also known as doping elements.
The doping element, i.e., the alkali, alkaline-earth or rare earth element, can be deposited on the alumina using any method which is known to the skilled person.
It can, for example, be carried out by impregnating the prepared alumina with doping elements or precursors of those elements, or by mixing the doping elements or precursors of those elements with the alumina during or prior to forming the latter. The doping element can also be deposited on the alumina by co-precipitation of the alumina and the doping elements or their precursors.
In the case of depositing by impregnation, this is effected in known manner by bringing the alumina into contact with a solution, a sol or a gel comprising at least one doping element in the form of an oxide or a salt or one of its precursors.
The operation is generally carried out by immersing the alumina in a set volume of a solution of at least one precursor of at least one doping element. The term xe2x80x9csolution of a precursor of a doping, elementxe2x80x9d means a solution of at least one salt or at least one compound of the doping element or elements, these salts and compounds being heat decomposable.
The concentration of the salt of the solution is selected as a function of the doping element to be deposited on the alumina.
The impregnation surface of the doping element is determined by the adsorbed volume of solution. Thus the adsorbed volume of the doping element is equal to the total pore volume of the alumina to be impregnated. It is also possible to impregnate the alumina by immersing it in the solution of the doping element precursor and eliminating the excess solution by draining.
In a preferred mode, the doping element is deposited by dry impregnation.
The compounds which can be deposited on the alumina can be selected from organic or inorganic compounds. They are preferably selected from inorganic compounds.
More particularly, the term xe2x80x9cinorganic compoundsxe2x80x9d means inorganic salts such as carbonates, bicarbonates, cyanides, cyanates, alkoxylates, hydroxides and nitrates.
The specific surface area of the composition after calcining, when the doping element is at least one element selected from the rare earths, is at most 250 m2/g, preferably at most 200 m2/g, more preferably at most 150 m2/g.
More particularly, the specific surface area of the composition after calcining, independently of the nature of the doping element, is at least 5 m2/g, preferably at least 10 m2/g, and more preferably at least 20 m2/g.
When depositing the compound comprising at least one element selected from alkalis, alkaline-earths and rare earths on the alumina, the concentration of the solution of the compound is selected such that the overall weight content of the alkali, alkaline-earth or rare earth elements in the composition is in the range 0.5% to 70% by weight with respect to the total composition weight.
Preferably, this amount is in the range 0.5% to 40% by weight, more preferably in the range 2% to 35% by weight, with respect to the total composition weight.
Without wishing to be bound by any particular theory, it can be considered that the substantial improvement in elimination of halogenated compound such as HCl and the substantial reduction or even suppression of the formation of halogenated oligomers in particular chlorinated oligomers known as green oils are probably due to the transformation of at least a portion of the alumina of the composition into an aluminate on calcining at the temperatures indicated above.
During calcining, at least partial inclusion of the compound(s) comprising the element(s) selected from the alkalis, alkaline-earths and rare earths is probable, leading to the formation of the corresponding aluminate.
The aluminate formed should be present in the superficial layers of the composition, which prevents the formation of Al-halogen entities, in particular Alxe2x80x94Cl entities, retained by the composition (the Al-halogen entity, in particular Alxe2x80x94Cl, is formed in the presence of alumina with a high specific surface area, which may be doped, and halogenated compounds, in particular chlorinated compounds contained in the stream), and as a result obstructs their reaction with unsaturated hydrocarbons to produce halogenated compounds, in particular chlorinated oligomers or green oils.
The process of the invention is applicable in all industrial fields requiring the elimination of halogenated compounds, in particular chlorinated compounds such as HCl and/or the reduction or even suppression of the formation of halogenated oligomers, in particular chlorinated oligomers known as green oils contained in a gas or a liquid primarily originating from regenerative catalytic reforming or from new generation.
The term xe2x80x9cregenerative catalytic reforming or new generationxe2x80x9d normally means processes characterized by being carried out at a pressure of at most 15 bars, more particularly at most 12 bars, for example 3 to 10 bars.
More particularly, the process of the invention is intended to eliminate chlorinated compounds, more particularly to eliminate the HCl present in a gas or a liquid, for substantial reduction and/or suppression of the formation of chlorinated oligomers or green oils also present in the gas or liquid.
As described above, at the outlet from the catalytic reforming, the gaseous effluents are mainly composed of hydrogen, saturated hydrocarbons, traces of unsaturated hydrocarbons, traces of halogenated compounds and water. When the effluents contain water, the water content is generally in the range 1 to 50 ppm by volume, at the pressure of the unit. Under these conditions, the HCl content, for example, is usually in the range 0.2 to 30 ppm by volume.
The process of the invention is also suitable for eliminating halogenated compounds and/or for substantial reduction or even suppression of the formation of chlorinated oligomers or green oils contained in a gas or a liquid which is free of water, or in a gas or liquid containing water.
The term xe2x80x9cfree of waterxe2x80x9d means a water content of less than 1 ppm, at the pressure of the unit.